The present invention relates to automatic chemical testing apparatus and more particularly to means for calibrating signal processing circuitry which provides output signals indicative of concentrations of substances in an analyzed sample.
A suitable environment contemplated for the present invention is disclosed in commonly assigned U.S. Pat. No. 3,728,079 issued Apr. 17, 1973 to John J. Moran, the disclosure of which is incorporated herein by reference. U.S. Pat. No. 3,622,279 issued Nov. 23, 1971 to John J. Moran, also commonly assigned and havint its disclosure incorporated herein by reference, is particularly directed toward means for providing readout signals in response to reacted contents in reaction containers.
In the type of apparatus under consideration, aliquots of a sample generally comprising human serum are each dispensed into a reaction container and reagents are added thereto. After sufficient incubation, a reading is made of the contents of the reaction container to provide a signal indicative of a particular parameter. In the preferred form, the reading is made spectrophotometrically, and an analog signal indicative of optical density is provided. In order to determine concentration of the particular substance for which analysis is being made of the particular aliquot, the optical density reading must be translated into a reading indicative of concentration units of a substance.
A means of providing an output indicative of concentration units is by the use of the electrical circuitry and printout means. The circuitry utilizes known techniques to process the analog readout signal indicative of optical density into an output signal for provision to printing or display means indicative of concentration units. The circuitry employs a linear curve relating optical density to concentration of the substance which has been empirically determined. (Curve fitting techniques are well-known in the art, and do not form a part of the present invention. Therefore, curve fitting techniques and circuitry are not discussed in great detail herein.)
However, in order to use the curves which have been provided by prior art curve fitting techniques successfully, reliable information must be provided for calibrating the curve. Calibrating the curve means taking an established curve to be utilized in a circuit and setting the zero intercept and slope thereof or providing a result equivalent thereto. Calibrating the curve is done by use of calibration samples. A sample called a blank is used to produce a signal which should result in an output signal at a zero or baseline level, and adjustment is made such that a proper output is thus produced. A sample called a reference having known concentrations of substances therein is measured, and the slope of the curve is adjusted so the output signal is at a proper readout value determined by the known value associated with the reference sample.
The function of calibrating output circuitry has been successfully performed in the past manually, for example, by means of operators observing meters indicative of output signals and performing manual potentiometer adjustments. A value based on reading the blank, or a blank value, and value based on a reading of the reference, or a reference value, form the basis for validity of outputs processed by the processing circuitry. In this manner, calibration values are provided to signal processing circuitry. While this form of adjustment is effective and reliable, it is even more desirable to provide a system capable of providing calibration values to prior art processing circuitry without operator intervention. It is further desirable to provide a system capable of periodically checking the validity of calibration values it is utilizing.